BONY SKIN PLATES 343
it has been discovered, are larger in the foetus, a fact which
naturally points to their being an inheritance from the past, HOW
undergoing retrogressive changes. Such a \vay of looking upon
the facts is confirmed by the finding, many years ago, by the
naturalist and physiologist Johannes Miller, of bony plates in
connexion with the remains of a Zenglodont Cetacean. It looks*
therefore, very much, as if the Eocene ancestors of the modern
Getacea had a skin studded with bony plates^ as have the arma-
dillos. This being the case, the disappearance of hair is not
surprising. The room, would lie taken up by the calcified plates,
and when the latter disappeared, as they have in the vast majority
of existing Whales, the naked skin alone would "be left,
"Whales possess no externally-visible hixi<i-lixnhs; rudiments
of these appendages present, which will be dealt with tinder
the description of the principal features of the skeleton. But
it lias been discovered that in the 1'orpoise, external vestiges
of hind-limbs do ap|>ear in the foetus^ a fact which, be It ob-
served,, does away with tlie old view that the flukes of the "Whale
are the last term in the series of vanishing hind-limbs* of which.
the Seals^ with their hind-limlte and tail bound up together,, offer
an intermediate step.
The tail is fish-like in form,, but the flukes are horizontal
instead of vertical as in fishes and Jishthyoaaur-us. This arrange-
ment is no doubt associated wifch the need for rapid return to
the surface waters after a prolonged immersion in search of food.
A downward stroke, such as is given by the powerful and large
tail flukes, would, naturally bring about this result rapidly.
The tail, moreover, is under all circumstances the swimming
organ. Its motion has been stated to be slightly rotatory, like
that of a scnew^ and it is the case that the two flukes are often
alternate in shape like the flanges of a screw; one being convex
upwards, the other convex downwards*
The fore-limbs are in the form of paddles, but they do not
apparently serve as organs of locomotion BO much as balancers.
"When a Whale is killed, it falls over on to one side, the office of
the flippers being to maintain the proper position. It is be-
lieved, however, from the fact that the embryo often shows a
relatively larger pectoral fin than that of the adult—the differ-
ence being due to a reduction in. the adult of the number of
phalanges—that the fin was one© an axgan of